Usgs preliminary assessment of volcanic and hydrothermal hazards in yellowstone national park and vicinity

Preliminary Assessment of Volcanic and Hydrothermal
Hazards in Yellowstone National Park and Vicinity
By Robert L. Christiansen, Jacob B. Lowenstern, Robert B. Smith, Henry Heasler, Lisa A. Morgan, Manuel
Nathenson, Larry G. Mastin, L. J. Patrick Muffler, and Joel E. Robinson
Open-file Report 2007–1071
U.S. Department of the Interior
U.S. Geological Survey

ii
U.S. Department of the Interior
DIRK KEMPTHORNE, Secretary
Mark D. Myers, Director
U.S. Geological Survey, Reston, Virginia 2007
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Suggested citation:
Christiansen, R. L., Lowenstern, J. B., Smith, R. B., Heasler, H., Morgan, L. A., Nathenson, M., Mastin, L. G.,
Muffler, L. J. P., and Robinson, J. E., 2007, Preliminary assessment of volcanic and hydrothermal hazards in
Yellowstone National Park and vicinity: U.S. Geological Survey Open-file Report 2007-1071, 94 p.
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Although this report is in the public domain, permission must be secured from the individual
copyright owners to reproduce any copyrighted material contained within this report.

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Contents
Contents................................................................................................................................................................iii
Brief Summary.......................................................................................................................................................1
Introduction ...........................................................................................................................................................4
Factors considered in this assessment................................................................................................................5
Current State of the Yellowstone system.............................................................................................................5
Geologic background ........................................................................................................................................5
Contemporary activity........................................................................................................................................8
Seismicity .......................................................................................................................................................8
Crustal deformation......................................................................................................................................11
Hydrothermal and gas activity.....................................................................................................................12
Pressures, temperatures, and fluids in geothermal systems .................................................................13
Mechanisms of hydrothermal explosion .................................................................................................14
Hydrothermal explosions in Yellowstone ................................................................................................15
Factors contributing to hydrothermal explosions ...................................................................................16
Toxic gases ...............................................................................................................................................17
The Hazards.........................................................................................................................................................17
Volcanic-eruption hazards..............................................................................................................................18
Basaltic eruptions ........................................................................................................................................18
Large rhyolitic eruptions..............................................................................................................................20
Small rhyolitic eruptions ..............................................................................................................................25
Large caldera-forming eruption...................................................................................................................26
Hydrothermal-explosion hazards....................................................................................................................29
How often do they occur?............................................................................................................................29
Potential effects ...........................................................................................................................................30
Precursory signals .......................................................................................................................................31
Where are hydrothermal explosions most likely to occur?........................................................................31
Seasonal and long-term effects on hydrothermal explosions ...................................................................33
Hazard mitigation .........................................................................................................................................33
Gas-emission hazards .....................................................................................................................................34
Relevant examples of toxic volcanic or hydrothermal gas hazards ..........................................................35
Hazard mitigation .........................................................................................................................................36
Conclusions .........................................................................................................................................................36
Acknowledgments...............................................................................................................................................37
References Cited.................................................................................................................................................37
Appendix 1. Description of representative historic hydrothermal explosions .................................................86
Porkchop Spring/Geyser .................................................................................................................................86
Excelsior Geyser..............................................................................................................................................86
West Nymph Creek Thermal Area ..................................................................................................................87
Black Opal/Wall Pool and Sapphire Pool .......................................................................................................87
Historic hydrothermal explosions elsewhere.................................................................................................87
Appendix 2. Description of large prehistoric hydrothermal eruption sites at Yellowstone .............................88
Pocket Basin....................................................................................................................................................88
Mary Bay..........................................................................................................................................................88
Elliott’s crater...................................................................................................................................................89
Evil Twin explosion crater ...............................................................................................................................89
Frank Island explosion crater..........................................................................................................................89
Indian Pond ......................................................................................................................................................89
Turbid Lake.......................................................................................................................................................90

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Appendix 3. Probabilities of episodic volcanic eruptions and application to the young intracaldera volcanic
history of Yellowstone.........................................................................................................................................91

1
Preliminary Assessment of Volcanic and
Hydrothermal Hazards in Yellowstone National Park
and Vicinity
By Robert L. Christiansen1
, Jacob B. Lowenstern1, Robert B. Smith2
, Henry Heasler3
, Lisa A.
Morgan1
, Manuel Nathenson1
, Larry G. Mastin1
, L. J. Patrick Muffler1
, and Joel E. Robinson1
Brief Summary
Possible future violent events in the active hydrothermal, magmatic, and tectonic system of
Yellowstone National Park pose potential hazards to park visitors and infrastructure. Most of the
national park and vicinity are sparsely populated, but significant numbers of people as well as park
resources could nevertheless be at risk from these hazards. Depending on the nature and magnitude
of a particular hazardous event and the particular time and season when it might occur, 70,000 to
more than 100,000 persons could be affected; the most violent events could affect a broader region
or even continent-wide areas. This assessment of such hazards is presented both as a guide for
future activities of the Yellowstone Volcano Observatory (YVO) and to aid appropriate response
planning by the National Park Service and surrounding agencies and communities. Although the
assessment is presented here in some technical detail, this summary is intended to be
understandable to non-scientists. The principal conclusions also will be made available in other
forms, more accessible to general readers.
The Yellowstone Plateau was built by one of Earth's largest young volcanic systems, having
episodically erupted great volumes of both lava and explosively ejected pumiceous ash for more
than 2 million years. These eruptive materials are products of two compositional types of
subsurface magma: basaltic magma is relatively fluid and, in this setting, generally produces small
to moderate volumes of lava in relatively brief eruptions; rhyolitic magma is more viscous and
either can erupt effusively to produce small to large volumes of lava or explosively to produce
coarse pumice and finer ash. The three largest Yellowstone eruptions produced blanketing deposits
of rhyolitic ash so hot that they welded into sheets of dense rock covering large areas, extending
beyond the national park. Each of them also produced a rain of ash that spread over much of
western and central North America and beyond; these ash deposits are greater than 2 m thick near
their eruptive sources and as much as a meter thick in surrounding areas. Each of these three
eruptions produced a caldera, or deep crater-like depression, tens of kilometers wide, formed by
collapse of the ground surface into a partly emptied subterranean magma chamber. The latest of
these three great eruptions formed the Yellowstone caldera. Renewed rhyolitic magma influx
beneath the Yellowstone caldera in central Yellowstone National Park uplifted parts of the caldera
floor and produced voluminous intracaldera lavas, the youngest of which extruded in a series of
eruptive episodes about 164,000, 152,000, 114,000, 102,000, and 72,000 years ago. During the
same span of time, generally smaller flows of both basalt and rhyolite have erupted in several areas
outside the Yellowstone caldera: (1) northwest of the caldera, (2) near the southern boundary of the
1
2
University of Utah
3
National Park Service

2
park, (3) in the basin of Island Park, west of Yellowstone National Park, and (4) especially in the
southern part of a corridor between Norris Geyser Basin and Mammoth Hot Springs.
Disruption of the Earth’s surface by faulting and regional uplift characterize the geologic
framework of Yellowstone Plateau volcanism. Some of the regional faults that bound the mountain
ranges around the Yellowstone Plateau are capable of producing large-magnitude (M>7)
earthquakes. In contrast, faults within the caldera are mainly small, produce smaller-magnitude
(M 6.8), relatively shallow earthquakes, and reflect strains in the Earth’s crust associated with
magmatic intrusion and hydrothermal activity. Swarms of generally small earthquakes occurring
within localized areas over restricted periods of time characterize much of the earthquake activity
within and adjacent to the Yellowstone caldera. This seismicity is monitored by a network of
seismographs within and adjacent to the park and is recorded and processed nearly in real time at
the University of Utah as part of the YVO program and archived as a contribution to the U.S.
Geological Survey (USGS) Advanced National Seismic System database.
Leveling surveys, satellite-based measurements, and geologic studies of former shorelines
of Yellowstone Lake all show that the entire area of the Yellowstone caldera and a seismically
active zone to the northwest undergo episodes of ground uplift and subsidence, sometimes
encompassing the entire caldera and sometimes in more local and complex patterns of both uplift
and subsidence. Such deformation in the Yellowstone region is monitored by YVO mainly through
a network of continuously recorded Global Positioning System (GPS) receivers recorded at the
University of Utah. The GPS data are incorporated as part of the National Science Foundation’s
EarthScope Plate Boundary Observatory, archived at UNAVCO and available through the YVO
website (http://volcanoes.usgs.gov/yvo/index.html).
The active hydrothermal system of Yellowstone National Park is one of the largest on
Earth. Although accidents involving hot water injure Yellowstone visitors from time to time,
conformance with normal Park Service procedures and regulations would ordinarily be sufficient to
prevent most of them. By contrast, a commonly recurring, more acute hazard at Yellowstone is the
explosive ejection of steam, water, and rock with no associated volcanism. These hydrothermal
explosions are caused by hot subsurface waters that flash to steam, breaking the overlying rocks
that confine them and ejecting the debris to form a crater. It is generally not clear just what triggers
these events, but possible triggers include strong local earthquakes, seasonal or long-term declines
in ground water levels, and changes in the underground distribution of heat. Many hydrothermal
explosions have few if any premonitory indications.
At least 26 hydrothermal explosions have been documented in the 126-year historic record
of the national park, and others undoubtedly escaped observation. Since the Yellowstone Plateau
was last glaciated, ending about 16,000 years ago, at least 18 large hydrothermal explosions have
formed craters wider than 100 m. Conservatively, at least one rock-hurling explosion every two
years is estimated to occur at Yellowstone, but because most of these events are small and usually
occur when few visitors are present, the likelihood of harm to park visitors is relatively small. The
average recurrence of an explosion that could cause personal injury is probably between 10 and 100
years. Average recurrence time of an explosion large enough to produce a 100-m-diameter crater is
probably about 200 years, but such an event could expel rocks and other hot debris more than 2 km
from the explosion site. Most hydrothermal-explosion craters at Yellowstone are in the Firehole
River geyser basins, beneath or around Yellowstone Lake, and in the southern part of the Norris-
Mammoth Corridor.
In addition to hydrothermal explosions, toxic gases—mainly carbon dioxide and hydrogen
sulfide—pose hazards. Concentrations of these gases in the atmosphere are generally low at
Yellowstone, but they can build up in confined areas such as valleys, caves, and tunnels, especially
during windless conditions. Most areas with toxic-gas hazards can be kept off-limits to people, but
gas emissions should continue to be monitored.

3
No volcanic eruption has occurred in Yellowstone National Park or vicinity in the last
70,000 years or more. Nevertheless, several types of volcanic eruption hazards are possible in the
future.
Basaltic lavas have erupted around the margins of the Yellowstone Plateau volcanic field
throughout its evolution. These relatively low-viscosity lavas generally erupt rapidly, most
eruptions lasting no more than a few weeks to a few months though the largest flow fields may
accumulate in multiple eruptions lasting months to years. The average period between basaltic
eruptions in the Yellowstone region since formation of the Yellowstone caldera has been about
16,000 years. The most likely location of a future basaltic eruption is within the basin of Island
Park, west of Yellowstone, but basalts could erupt anywhere in a 40-km-wide band around the
caldera. Future basaltic eruptions could cover several square kilometers with lava up to tens of
meters thick. Basaltic ash and cinders also might blanket hundreds of square kilometers to depths
of a few meters to a few centimeters, and if a vent emerged beneath water or saturated ground,
more explosive eruptions could cause significant destruction, such as blasting down trees or
structures.
Large rhyolitic lava flows, many having volumes greater than 10 km3
, have erupted within
the Yellowstone caldera during the past 170,000 years. Initially these larger eruptions were
preceded by explosively ejected pumice and ash. In a similar future eruption, ejecta could bury
broad areas, locally to many meters. Subsequent lava extrusion could last for years, covering areas
as great as 350-400 km2
to thicknesses of tens or hundreds of meters and volumes of 5 to more than
50 km3
. Because such voluminous rhyolitic lava eruptions have not been observed anywhere in
historical time, it is uncertain how long such an event might continue; extrusion might be orders of
magnitude faster than for smaller flows. The probability of a future large intracaldera rhyolitic
eruption is difficult to estimate. Available data suggest a highly episodic behavior of past eruptions
of this sort, periods of a few thousand years characterized by numerous eruptions being separated
by longer intervals of about 12,000 to 38,000 years without eruption. One statistical measure of
eruption probabilities based on this episodic behavior suggests an average recurrence of 20,000
years. The fact that no such eruption has occurred for more than 70,000 years may mean that
insufficient eruptible magma remains beneath the Yellowstone caldera to produce another large-
volume lava flow.
Small rhyolitic lava flows postdating the Yellowstone caldera have erupted mainly north of
the caldera, but one such flow also lies near the South Entrance to the park. Two distinct types of
primary hazards might be associated with small rhyolitic eruptions at Yellowstone. Just as for
larger rhyolitic lava eruptions, initial venting almost certainly would explosively eject rhyolitic
pumice; the coarser fragments would fall back close to the vent, but finer pumiceous ash would
enter the atmosphere and fall downwind for many kilometers. Structures, power lines, etc. could be
damaged by ash loading, especially if eruption were accompanied by heavy rain. The initial
explosive eruptions could last a few hours to several weeks and be followed by viscous extrusion of
rhyolitic lava, covering several square kilometers to tens of meters thick; lava could continue to
extrude for many months or even years. Viscous rhyolitic lava would advance much more slowly
than a basaltic flow; most affected facilities could be safely evacuated and perhaps relocated. The
average recurrence period of small extracaldera rhyolitic eruptions in the Yellowstone Plateau
volcanic field is about 50,000 years.
In addition to the primary hazards posed by any future eruption of basalt or a small or large
rhyolitic lava eruption, important possible secondary consequences include wildland fires, debris
flows, and floods triggered by the displacement of surface drainages by lava.
Systematic seismic, deformation, and hydrothermal monitoring by YVO is likely to provide
indicators of any impending volcanic eruptive activity in Yellowstone National Park. Premonitory
events detected by such monitoring might include multiple shallow earthquake swarms of

4
increasing frequency and intensity, the ground vibrations called volcanic tremor, localized uplift of
the surface, ground cracks, and anomalous gas emissions.
Of all the possible hazards from a future volcanic eruption in the Yellowstone region, by far
the least likely would be another explosive caldera-forming eruption of great volumes of rhyolitic
ash. Abundant evidence indicates that hot magma continues to exist beneath Yellowstone, but it is
uncertain how much of it remains liquid, how well the liquid is interconnected, and thus how much
remains eruptible. Any eruption of sufficient volume to form a new caldera probably would occur
only from within the present Yellowstone caldera, and the history of postcaldera rhyolitic eruptions
strongly suggests that the subcaldera magma chamber is now a largely crystallized mush. The
probability of another major caldera-forming Yellowstone eruption, in the absence of strong
premonitory indications of major magmatic intrusion and degassing beneath a large area of the
caldera, can be considered to be below the threshold of useful calculation.
Introduction
Yellowstone National Park, justly famous for its unmatched geysers, diverse wildlife, and
uniquely preserved ecologic communities, also encompasses one of Earth’s largest systems of
volcanic, seismic, and hydrothermal activity. In recognition of the importance of this active Earth
system, officials of the U.S. Geological Survey (USGS), Yellowstone National Park, and the
University of Utah in May of 2001 jointly established the Yellowstone Volcano Observatory
(YVO). The stated objectives of this new observatory are: (1) to provide monitoring that enables
reliable and timely warnings of possible renewed volcanism and related hazards in the Yellowstone
region, (2) to notify National Park and other local officials and the public of any significant local
seismic or volcanic events, (3) to improve scientific understanding of the fundamental tectonic and
magmatic processes that create the Park’s ongoing seismicity, surface deformation, and
hydrothermal activity, (4) to assess the long-term potential hazards that volcanism, seismicity, and
explosive or other convulsive hydrothermal activity might pose to the park and its surroundings, (5)
to communicate as effectively as possible to responsible authorities and the public the results of
scientific monitoring and hazard-assessment activities, and (6) to improve coordination and
cooperation among the three institutions responsible for YVO. The observatory is built upon a
substantial base of previous cooperative work among these institutions and seeks to assure a solid
long-term basis for the continuity and improvement of scientific monitoring of the Yellowstone
magmatic-tectonic-hydrothermal system.
The hazard assessment presented in this report is intended to help guide future activities of
the observatory as well as to provide a basis for appropriate management actions by the National
Park Service and other agencies in the Yellowstone area in the event of any future hazardous events
that might result from activity of the Yellowstone system. The assessment is part of an ongoing
three-part process that was set in motion at the outset of YVO’s work. The first part of the process
was a comprehensive review of basic scientific knowledge of Yellowstone’s magmatic-tectonic-
hydrothermal system. Such a review, of course, is never final and must be continually reexamined
and revised, particularly in the light of current monitoring data. The second step of the process is
assessment of the relevant hazards. The current hazards assessment is intended to be fully
documented and scientifically defensible; because this necessarily entails a degree of scientific
rigor and technical documentation not readily understandable by general readers or to all concerned
individuals, additional reports more suitable for non-scientist readers also are part of the ongoing
assessment activity. One general-interest publication on Yellowstone’s volcano, seismic, and
hydrothermal hazards has already been published (Lowenstern and others, 2005). The final step of
the process will be a response plan, based upon the conclusions of this assessment, by the
responsible Yellowstone National Park officials, in cooperation with other appropriate agencies and
with the assistance of observatory scientists.

5
It is worth noting here that, despite the fact that most of Yellowstone National Park and its
nearby surroundings are sparsely populated, many thousands of people as well as National Park
resources could be at risk from the types of hazards considered in this assessment. About 3 million
people visit Yellowstone each year, principally during the three summer months. A similar number
of people visit adjacent Grand Teton National Park (although some of these are the same visitors).
Including residents of the surrounding communities of Wyoming, Idaho, and Montana, the daily
population exposure to these hazards during the summer months could average between 70,000 and
more than 100,000 persons. This exposure produces a largely unappreciated level of risk,
comparable to that of other areas having considerably larger resident populations. Furthermore, the
largest of these hazards, although they have the lowest probabilities of occurrence, could affect
much of western and central North America. Indirect effects, especially on climate, could be
global.
Factors considered in this assessment
In order to clarify the intended scope of this report, it is important to state at the outset just
what factors are considered explicitly in this assessment. Hazards that might result from ongoing
or future activity of the Yellowstone magmatic-tectonic-hydrothermal system, particularly volcanic
eruptions, earthquakes, and hydrothermal eruptions, are the focus of this discussion. Certain other
geologic hazards, although likely to be of important concern to National Park Service managers in
the future, are not included within its scope. Examples of the latter would include landslides,
debris flows, or floods, except insofar as they are considered here as possible consequential
secondary hazards that might result from major volcanic, seismic, or hydrothermal activity.
The area considered in this hazard assessment is primarily Yellowstone National Park (fig.
1). Nevertheless, because most of the hazards considered here could have significant effects on
adjacent communities, the Park boundary does not constitute an absolute limit in our analysis.
The strategy is to establish an integrated view of the sources of relevant hazards and some
scenarios for how they might develop in time. This approach diverges from that of many previous
USGS volcano-hazard assessments in not emphasizing a catalogue of individual hazardous
processes and zonation maps for each of them. Rather, the emphasis is on (1) what kinds of
monitoring data might be of immediate concern to National Park Service managers or might be
considered premonitory to hazardous events, (2) the probabilities of recurring volcanic and acute
hydrothermal events, and (3) how multiple events might be related to one another. Although the
present open-file version of this report does not analyze earthquake hazards explicitly, a
forthcoming revision for more formal publication will include seismic hazards.
Current State of the Yellowstone system
Assessment of possible future activity in Yellowstone starts from an analysis of the current
state of the magmatic-tectonic-hydrothermal system. We first review the geologic framework and
follow with information on current activity and monitoring of the system.
Geologic background
The Yellowstone Plateau volcanic field of Wyoming, Idaho, and Montana (fig. 2) is one of
Earth's largest young volcanic systems, having erupted extraordinarily voluminous rhyolites
episodically over a little more than 2 million years (Christiansen, 1984; 2001). Its three largest
eruptions deposited sheets of mainly welded ash-flow tuffs regionally as well as coeval ash-fall
layers that fell over much of western and central North America (fig. 3). The volcanic field is the
current expression of a major sublithospheric mantle source that generates a melting anomaly that

6
has propagated northeastward at least 300 km relative to the North America craton since its initial
volcanism at 17-14 Ma (fig. 4). Researchers debate the nature of this melting source—whether, for
example, it represents a convective thermal plume from the base of the mantle or from the mantle
transition zone, an entirely upper-mantle response to plate-tectonic processes, or some other
mechanism (e.g., Pierce and Morgan, 1992; Smith and Braile, 1993, 1994; Pierce and others, 2000;
Christiansen and others, 2002; Camp and Ross, 2004; Waite and others, 2006). These varied
hypotheses are significant to interpretation of regional geophysical data but have only an indirect
bearing on the present analysis of volcanic hazards.
The volcanic field has evolved episodically in three cycles of rhyolitic activity. Each cycle
culminated in the rapid eruption of voluminous rhyolitic ash flows—hundreds to thousands of
cubic kilometers—and consequent catastrophic subsidence of the source areas to form large
calderas. Each climactic ash-flow eruption was preceded by a period of magmatic intrusion and
intermittent rhyolitic lava eruptions and was followed by a period of partial filling of the calderas
with rhyolitic lavas. During each of these cycles, basaltic lavas erupted around the margins of the
active rhyolitic source area but not within it. About a million years after their rhyolitic activity,
basalts finally erupted through the source areas of the first two cycles, but no basalts have yet
erupted within the youngest, the Yellowstone caldera.
Collectively, the voluminous rhyolitic ash-flow tuffs of the volcanic field are known
stratigraphically as the Yellowstone Group (Christiansen and Blank, 1972). The oldest and largest
of the caldera-forming eruptions produced the Huckleberry Ridge Tuff at 2.059±0.004 Ma
(Lanphere and others, 2002), covering an area of more than 15,000 km2 with ash flows having a
cumulative volume of at least 2,450 km3. The resulting caldera (fig. 5, purple line) spanned nearly
the entire width of the Yellowstone Plateau volcanic field (Christiansen, 1979, 2001). The Mesa
Falls Tuff of 1.285±0.004 Ma (Lanphere and others, 2002), smallest of the three ash-flow sheets,
erupted in the Island Park area west of the Yellowstone Plateau (Christiansen, 1982, 2001) and is
now exposed mainly in that vicinity, adjacent to its source caldera (fig. 5, blue line). The Mesa
Falls probably initially covered an area of more than 2,700 km2 and had an eruptive volume of
more than 280 km3. The youngest of the major caldera-forming eruptions at 0.639±0.002 Ma
(Lanphere and others, 2002) produced the Lava Creek Tuff and the Yellowstone caldera, now the
central feature of the volcanic field (Christiansen, 1984). Two distinct parts of the Lava Creek Tuff
erupted from separate segments of the caldera (Christiansen, 1979, 2001) but form a single
compound cooling unit that initially covered at least 7,300 km2 and had an eruptive volume of at
least 1,000 km3.
Precaldera rhyolitic lavas of the third volcanic cycle erupted from a growing ring-fracture
system in the area that subsequently became the Yellowstone caldera. Eight such precaldera
rhyolite flows are known, yielding isotopic ages from 1.22±0.01 to 0.609±0.006 Ma (table 1; see
note on the table regarding the accuracy of cited ages). Additional precaldera rhyolitic lava flows
may now lie entirely buried within the Yellowstone caldera. The precaldera lavas, named the
Mount Jackson Rhyolite and Lewis Canyon Rhyolite, and the growing ring-fracture system from
which they extruded (fig. 6) are interpreted as indicating magmatic intrusion and the growth of a
large rhyolitic magma chamber in the shallow crust during a period of nearly 600,000 years beneath
the area that would later erupt catastrophically to produce the Lava Creek Tuff and subside to form
the Yellowstone caldera (fig. 7).
Shortly following the climactic caldera-forming Lava Creek eruption, rhyolitic magma
again intruded the subcaldera region, uplifting segments of the caldera floor in a pair of resurgent
domes bounded by two inner ring-fracture segments enclosed by an outer ring-fracture zone
encompassing nearly the entire Yellowstone caldera (Christiansen, 2001). The western of these
resurgent domes is the Mallard Lake dome (fig. 7, ML); the eastern is the Sour Creek dome (fig. 7,
SC). Subsequently, additional rhyolitic lavas have erupted within the caldera during several

7
volcanic episodes. These rhyolites, mainly constituting large lava flows, are included
stratigraphically in the Plateau Rhyolite. The oldest of the Plateau Rhyolite flows, the Upper Basin
Member, erupted within the caldera from around the inner ring-fracture zones that bound the two
resurgent domes (fig. 7). These oldest postcaldera rhyolites yield isotopic ages that range between
516±7 and 479±10 ka (Gansecki and others, 1996) though there is a reasonable possibility that their
actual eruptive ages were even closer to that of the ~639-ka Lava Creek Tuff (see Lanphere and
others, 2002). Buried rhyolitic lavas identified in recent high-resolution sonar and seismic-
reflection mapping of Yellowstone Lake (Morgan and others, 2003b) and found as lithic clasts in
postglacial hydrothermal-explosion deposits south of the Sour Creek dome and in northern
Yellowstone Lake yield a 40
Ar/39
Ar age of 600±20 ka (Morgan and Shanks, 2005). Additional
postcaldera lava, probably as young as 275±11 ka is known (table 1). Other lavas may well have
extruded within the caldera during the time before about 170 ka but remain buried beneath younger
lavas and sediments.
Younger postcaldera rhyolitic volcanism dates from about 170 ka (Christiansen, 2001).
This activity probably began with extrusion of rhyolitic lava onto the Mallard Lake resurgent dome
in the western part of the Yellowstone caldera (the Mallard Lake flow) followed by renewed uplift
of that dome. Following those events, as much as several hundred cubic kilometers of rhyolitic
lava (figs. 8 and 9), named the Central Plateau Member of the Plateau Rhyolite, has nearly filled
the Yellowstone caldera from eruptive vents along two linear northwest-trending zones that project
across the caldera from neighboring tectonic fault zones. The western of these two zones extends
from the Teton fault zone (see fig. 11) to the tectonic basin of West Yellowstone. Lavas from these
vents form the Pitchstone and Madison Plateaus (fig. 1) and bury the western rim of the
Yellowstone caldera. The eastern zone extends northward across the caldera from the Sheridan
fault zone (see fig. 11) to the Norris Geyser Basin and erupted the lavas that form the Central
Plateau. The lava flows from this youngest intracaldera activity are typically quite large—some of
them exceeding 20 km3—and appear to have erupted in five episodes at about 164±5, 152±3,
114±2, 102±5, and 72±4 ka (table 2 and figs. 8 and 9). It is likely that rhyolitic pumice and ash
erupted during the opening of vents for each of these lava flows. In addition, concurrent with lava
eruptions of the Central Plateau Member, at least two significant pyroclastic eruptions occurred
within the caldera, depositing the tuffs of Bluff Point and Cold Mountain Creek. The former was
of sufficiently large volume to have caused subsidence of its source area to form a smaller caldera
(~10-km diameter) within the Yellowstone caldera, now partly preserved as West Thumb (fig. 8),
the western part of Yellowstone Lake (Christiansen, 2001). It is worth noting that this relatively
small caldera is itself as large as the well-known caldera of Crater Lake, Ore., which formed as a
result of the pyroclastic eruption of more than 50 km3
of magma (Bacon, 1983).
Volcanism postdating the Yellowstone caldera also has occurred in areas outside the caldera
but within or near the boundaries of Yellowstone National Park. The individual eruptive products
of this extracaldera volcanism are generally smaller in volume than most of those within the caldera
but span nearly the same range of time as the intracaldera lavas. Both rhyolites and basalts occur
among these extracaldera lavas. K/Ar and 40
Ar/39
Ar ages obtained on the rhyolites range between
526±3 and 80±2 ka (table 1). Basaltic eruptions appear to have spanned much of the same time.
Basalts and rhyolites both erupted mainly in the southern part of the Norris-Mammoth corridor, a
zone of faulting, volcanism, and hydrothermal activity that extends northward from the caldera
margin near Norris Geyser Basin to just north of Gardiner, Mont. (figs. 10, 11). Other rhyolites and
basalts of similar ages have erupted northwest of the caldera, between the Madison River and
Cougar Creek, and near the southern boundary of Yellowstone National Park. Furthermore, basalts
erupted from at least 17 vents in the basin of Island Park, within 35 km of the west boundary of
Yellowstone National Park (fig. 10).

8
Hydrothermal activity occurs widely on and around the Yellowstone Plateau. Most of the
hottest and most active areas of geysers and other near-neutral (i.e., non-acidic) hot springs occur
within topographic basins situated above the ring-fracture zone of the Yellowstone
caldera—including Yellowstone Lake (Johnson and others, 2003; Morgan and others, 2003a;
Morgan and others, 2003b)—but others occur in the Norris-Mammoth corridor north of the caldera.
Other major sites of mainly acidic, sulfate-rich hot-springs and fumaroles occur in topographically
higher areas in and adjacent to the ring-fracture zone and the northeastern caldera rim. Smaller,
less concentrated zones of hydrothermal activity are distributed widely around the Yellowstone
National Park, but most areas deemed capable of producing violent events such as hydrothermal
explosions are in the major hydrothermal areas within or adjacent to the caldera and in the Norris-
Mammoth corridor.
Regional uplift and normal faulting have established the geologic framework within which
the Yellowstone Plateau volcanic field lies (figs. 4 and 11). Faults mapped within the caldera are
mainly small and seem to reflect strains associated with magmatic intrusion and, perhaps, shallow
hydrothermal activity. The principal faults south of the caldera that accommodate regional tectonic
extension generally trend nearly north-south and define the Teton, Sheridan, Flat Mountain, and
Upper Yellowstone fault zones. Major fault zones north of the caldera include the Lamar, East
Gallatin-Washburn, Hebgen, and Madison Valley fault zones, many of them trending more nearly
northwest. A little farther west, the Centennial fault zone trends east-west. The somewhat arcuate
Mirror Plateau fault zone northeast of the caldera (figs. 6 and 11) appears to have accommodated
both regional tectonic extension and displacements associated directly with the Yellowstone
caldera system (Christiansen, 2001).
Bathymetric and seismic-reflection studies have delineated several small faults beneath
Yellowstone Lake (Otis and others, 1977; Wold and others, 1977; Johnson and others, 2003;
Morgan and others, 2003b). At least the major sublacustrine faults appear to represent
continuations of the regional tectonic trends noted above (Morgan and others, in press-b).
Thermal surveys of Yellowstone Lake (Morgan and others, 1977), together with
geochemical indicators of the heat output of the Yellowstone hydrothermal system (Fournier,
1989), demonstrate that the total heat flux from the Yellowstone caldera exceeds 1500 mW/m2
,
more than thirty times the regional average. This great thermal flux is a direct reflection of the
magmatic heat source that produces and sustains Yellowstone’s hydrothermal activity.
Contemporary activity
The tectonic-volcanic-hydrothermal system of the Yellowstone region is vigorously active.
The hydrothermal system may be the largest on Earth, and the subsurface magmatic system
continually deforms the ground surface on time scales of months to a few years. Regional tectonics
and the magmatic system combine to produce some of the highest levels of earthquake activity in
the conterminous U.S. outside of California.
Seismicity
Although physiographically part of the Middle Rocky Mountains, Yellowstone lies along
the northeastern margin of the extensional basin-range tectonic region (figs. 4 and 11). Epicenters
of earthquakes associated with this tectonic extension define a belt of seismicity that extends north
from the Wasatch front in Utah to the Yellowstone Plateau, then branches to the northwest and
west into Montana and Idaho, termed the intermountain seismic belt by Smith and Sbar (1974).
Earthquake epicenters define a parabolic arc around the north, east, and south sides of the eastern
Snake River Plain through the Yellowstone Plateau, where the seismicity is most active (Smith and
Arabasz, 1991; Smith and Braile, 1994) and fault displacements are greatest (Anders and others,

9
1989; Pierce and Morgan, 1992). Earthquake distributions, focal mechanisms, and GPS
determinations of crustal strain in the Yellowstone Plateau area are all consistent with the generally
NE-SW direction of extension in the basin-range region (Waite and Smith, 2004; Puskas and
others, 2007).
Seismicity in the Yellowstone National Park region is monitored in real time by a network
of 26 seismic stations within or adjacent to the park (fig. 12), all of which are recorded at the
University of Utah as an integral part of the YVO program and archived in the Advanced Seismic
Network System. Of these seismic stations, 17 are single-component short-period seismometers
connected to the network by analog telemetry. Additionally, there are three short-period 3-
component seismometers with analog telemetry and six broadband 3-component seismometers with
digital telemetry (Yellowstone Volcano Observatory, 2006). Data are transmitted continuously
from the seismic stations via radio signals to a Federal Aviation Administration (FAA) radar site
located on Sawtell Peak, Idaho, west of Yellowstone National Park, where the signals are
multiplexed onto four FAA microwave lines for retransmission to the FAA control center in Salt
Lake City, Utah. From there, the data are transmitted to the University of Utah's central recording
laboratory via telephone lines. Earthquake data are processed using the USGS EarthWorm system
to produce automated real-time hypocenter determinations and emergency-response broadcasts.
Refined earthquake locations, magnitudes, and focal mechanisms are produced by seismic analysts
at the University of Utah Seismograph Stations from the digital data and are provided to YVO
users.
Given its average station spacing of 15-30 km and seismometer characteristics, the
Yellowstone network is optimally capable of detecting local earthquakes of magnitudes greater
than about -1.0 and locating their epicenters within about 1 km and their focal depths within about
0.5 km at depths below about 4 km (or about 1 km at shallower depths). Analysis of the digital
seismic data includes production of a revised catalog of Yellowstone earthquakes using new three-
dimensional P-wave velocity models determined by tomographic inversion of local earthquakes
(Husen and Smith, 2004) and a new magnitude scale for improved hazard assessments.
It is useful to consider seismicity of the Yellowstone region in terms of several categories of
activity. Most Yellowstone earthquakes (fig. 13) are of small magnitude (M 3), but a few large
tectonic earthquakes have affected the region, including the deadly Hebgen Lake earthquake of
August 17, 1959, with a magnitude of 7.5 (U.S. Geological Survey, 1964; Trimble and Smith,
1975; Doser, 1985; Doser and Smith, 1989). Aftershocks of the Hebgen Lake earthquake were
numerous (Murphy and Brazee, 1964), and a high proportion of the earthquakes since then have
occurred in the same belt of seismicity as those early aftershocks (Trimble and Smith, 1975; Smith
and others, 1977; Smith and Braile, 1994). Numerous other earthquakes, including many within
the Yellowstone caldera, are scattered more widely, are typically shallow, and commonly occur in
the form of seismic swarms. Seismicity beneath the Yellowstone caldera is generally shallower (~5
km) relative to the deepest earthquakes (~20 km) on tectonic faults outside the caldera, probably
related to the effects of elevated crustal temperature above the magma that underlies the caldera
(Miller and Smith, 1999; Waite and Smith, 2002; Husen and Smith, 2004). Thus, seismicity can be
discussed in four categories: large earthquakes on regional tectonic faults, smaller tectonic
earthquakes, intracaldera earthquakes, and seismic swarms.
The 1959 Hebgen Lake earthquake had the largest magnitude of any historic earthquake in
the Rocky Mountains region (fig. 13). That earthquake produced a pair of fault scarps up to 6 m
high and 40 km long, caused a landslide that dammed the Madison River northwest of Hebgen
Lake about 30 km west of Yellowstone National Park, caused extensive damage to roads and
buildings within the park, and resulted in 28 deaths—all of them in the National Forest, outside the
park. The only other regional historic earthquake of roughly comparable magnitude was the M=7.3
Borah Peak, Idaho, earthquake of 1983, about 150 km southwest of Yellowstone National Park

10
(Dewey, 1983; Smith and others, 1985; Crone, 1987; Richins and others, 1987). It too produced a
long surface scarp and resulted in local damage. Both of these earthquakes caused discernable
changes in the hydrothermal features of Yellowstone (Watson, 1961; Marler, 1964; Marler and
White, 1975; Hutchinson, 1985). Prehistoric late Holocene scarps occur on several range fronts
near Yellowstone, including the Centennial, Madison, and Teton Ranges (pl. 1 of Pierce and
Morgan, 1992), indicating the possibility of further large tectonic earthquakes in the region.
Smaller tectonic earthquakes on regional faults occur widely in areas in and adjacent to
Yellowstone National Park (Smith and Sbar, 1974; Doser and Smith, 1983; Waite and Smith, 2002,
2004; White, 2006). Recurring seismicity of this type is common and must be expected in the
future. Most of these earthquakes are small and cause no significant damage. Others are felt
locally, and some have the potential to cause damage to structures or other facilities.
Intracaldera earthquakes are typically small (M 3) and generally are shallower than about 6
km (Miller and Smith, 1999; Husen and Smith, 2004; Waite and Smith, 2004). A few however, are
larger and can be damaging. The largest earthquake recorded within the caldera area occurred
southeast of Norris Geyser Basin (fig. 13) in June, 1975 with a magnitude of 6.1 (Pitt and others,
1979). The earthquake, felt widely within Yellowstone National Park, caused damage to roads and
other visitor facilities. The area in which it was felt was more extensive to the north, outside the
caldera, than within the caldera, probably because of the attenuating effects of elevated crustal
temperatures beneath the caldera. Aftershocks occurred within a zone 10 km long that included the
main shock as well as on a parallel zone of seismicity about 5 km to the west that had been
intermittently active for the preceding year.
As is common in other volcanic and geothermal areas, many of the earthquakes in or
adjacent to the Yellowstone caldera occur in swarms (Smith and Braile, 1994; Waite and Smith,
2002; Farrell, 2007),. These swarms are characterized by numerous small, generally shallow
earthquakes clustered in both space and time, without a mainshock, and having magnitude
differences 0.25. Commonly these swarms are aligned on tectonic trends, though not necessarily
on recognized faults, and are generally considered to be related to migration of hydrothermal fluids
or magma in the shallow crust. A particularly intense earthquake swarm (fig. 14) occurred in
October and November of 1985 a short distance northwest of the Yellowstone caldera (Waite and
Smith, 2002). With time, the swarm activity migrated both away from the caldera and, to a lesser
extent, downward as the caldera itself underwent a change from uplift to subsidence. The swarm
was likely to have been related to the subsurface movement of hydrothermal fluids or magma from
beneath the caldera, deflating the caldera area and possibly penetrating the shallow crust nearby as
magmatic dikes that did not breach the surface in eruption (Waite and Smith, 2002). Possible
migration of magma may have induced local migration of hydrothermal fluids and magmatic gases
(Husen and others, 2004).
Induced or triggered earthquakes constitute another class of seismicity observed recently in
Yellowstone (Husen and others, 2004). Such earthquakes are triggered by the passage of large-
amplitude surface waves from distant sources to produce local transient dynamic stress changes as
great as 0.15 MPa. A series of local earthquake swarms was observed, and clear changes in geyser
activity occurred immediately following local arrival of seismic waves from the 2002 Denali fault,
Alaska, earthquake (M=7.9) in the Yellowstone National Park area, 3100 km from the epicenter.
Beginning within hours of the arrival of surface waves, the YVO network located more than 250
earthquakes in the first days after the Denali event. The eruption frequency of several geysers was
altered, and numerous earthquake swarms close to major geyser basins were unusual in occurring
simultaneously in different geyser basins. This behavior is interpreted as having been induced by
dynamic stresses associated with the passage of large-amplitude surface waves, perhaps locally
altering the permeability of hydrothermal plumbing by unclogging existing fractures. Furthermore,
redistribution of hydrothermal fluids and locally increased pore pressures triggered earthquakes in

11
these swarms. Although earthquake triggering and changes in geyser activity have been
documented elsewhere, these notable changes at Yellowstone induced by a large-magnitude event
at such a great distance indicate the profound effects that remotely triggered seismicity can have on
hydrothermal and earthquake hazards and also suggests that earthquakes and hydrothermal
explosions at Yellowstone might be triggered by local earthquakes in the future.
Crustal deformation
Major surface deformation has been recognized in Yellowstone since precise leveling
surveys were repeated in 1975-1977 after a lapse of 50 years. Caldera-wide uplift to a maximum of
at least 73 cm (fig. 15) was documented in these surveys (Pelton and Smith, 1979, 1982). Similar
uplift continued, as measured repeatedly in several following years (fig. 16), averaging 23 mm/y
from 1976 to 1983, then slowing down for about a year before subsiding as much as 35 mm/y until
1987 (Dzurisin and Yamashita, 1987; Dzurisin and others, 1990).
Since these surveys, newer methods, including the Global Positioning System (GPS) and
Interferometric Synthetic Aperture Radar (InSAR) have shown that surface deformation remains
active over the entire area of the Yellowstone caldera and the seismically active area to the
northwest, including the epicenter of the 1959 Hebgen Lake earthquake. Both uplift and
subsidence occur in this area, sometimes in a caldera-wide fashion and at other times in more
complex patterns of more local uplift and subsidence (Meertens and Smith, 1991; Wicks and
others, 1998; Puskas and others, 2002; Wicks and others, 2006; Puskas and others, 2007).
Currently, deformation monitoring of the Yellowstone region is carried out by YVO mainly
through a network of 16 GPS receivers (Yellowstone Volcano Observatory, 2006), of which 12 are
located within Yellowstone National Park (fig. 17), continuously recorded at the University of Utah
and by the EarthScope Plate Boundary Observatory. Additional stations are planned for future
installation. Each station records a time series of 2 horizontal components and 1 vertical
component sampled every 5 seconds, which are analyzed at the University of Utah to produce
coordinate solutions. In addition, about 60 GPS stations are recorded in special surveys rather than
being monitored continuously. Repeat leveling surveys also continue intermittently.
Together, the geodetic data reveal complex patterns of crustal deformation over a period of
decades (fig. 14), as summarized by Puskas and others (2007). From 1923 to 1985 caldera-wide
uplift occurred at an average rate of 22 mm/yr, changing in 1986 to subsidence. Subsidence
continued within the Yellowstone caldera to 1995 at 14 mm/yr while uplift occurred northwest of
the caldera at 5 mm/yr. In 1995, caldera uplift resumed at 9 mm/yr, changing again in 2000 to
subsidence at 9 mm/yr while uplift continued northwest of the caldera at an average of 12 mm/yr.
A sudden change in 2004 to caldera uplift proceeded at unprecedented rates of up to 60 mm/yr,
continuing into 2007. These rapid changes in motion across the Yellowstone caldera over more
than a half century clearly reflect the importance of transient large-scale crustal deformation
sources that probably include both the subcaldera magma chamber and localized zones of
pressurized hydrothermal fluids that inflate and deflate the surface as the locations and properties of
the sources change with time.
Detailed analysis of ancient shorelines of Yellowstone Lake, both subaerial and
sublacustrine, indicates that similar changes of major uplift and subsidence date back at least as far
as the final retreat of Pleistocene glaciers from the lake basin (Meyer and Locke, 1986; Locke and
Meyer, 1994; Pierce and others, 2002; Johnson and others, 2003). Pierce and colleagues (2002)
conclude that cycles of uplift and subsidence have occurred repeatedly in the Yellowstone Lake
basin during the past 14,000 years; the record of these cycles is superposed on the record of a long-
term decrease in lake level by downcutting of the lake outlet, but that little net elevation change
occurred between about 14 ka and 3 ka (fig. 18).

12
Together, earthquake focal mechanisms and both vertical and horizontal deformation
patterns suggest that the crustal stress field recorded both by earthquakes and by surface
deformation are generally consistent with regional basin-range extension. In the vicinity of the
Yellowstone caldera, however, the stress field is strongly modified by the effects of migrating
magmatic and hydrothermal fluids to produce changes in seismicity, uplift, and subsidence on time
scales of months to a few years (Dzurisin and others, 1990; Dzurisin and others, 1994; Puskas and
others, 2002; Waite and Smith, 2002; Husen and others, 2004; Waite and Smith, 2004; Wicks and
others, 2006; Puskas and others, 2007).
Hydrothermal and gas activity
At each eruption, immediately preceding, was an upheaval of some fifty feet high, followed by
one great explosion in which the water was thrown two hundred fifty to three hundred feet and
frequently hurling stone one foot in diameter five hundred feet from the crater.
Joaquin Miller (in Muir, 1888), describing hydrothermal explosions of Excelsior Geyser.
Yellowstone National Park hosts one of the largest hydrothermal systems on Earth and, in
particular, has more geysers than all others in the world together. Most of these hot springs,
geysers, and fumaroles present hazards only to visitors who may stray from established pathways
into areas of unstable ground and hot water; it is impractical to consider such daily hazards in the
context of the present assessment. One of the most common acute geological hazards at
Yellowstone, however, consists of shallow-rooted explosions of steam, water, and rock without
any associated volcanism. These hydrothermal explosions (or eruptions) occur when hot
subsurface waters flash to steam, violently breaking the confining rocks and ejecting them from a
newly formed or existing crater. Individual events can last as briefly as a few seconds or as long as
several hours, and intermittent explosive activity can continue for years.
Hydrothermal-explosion craters are found in Yellowstone National Park at scales ranging
from less than a meter to several kilometers in diameter (Muffler and others, 1971; Morgan and
others, in review). Visitors to popular thermal areas like Upper and Lower Geyser Basins, Norris
Geyser Basin, and West Thumb Geyser Basin see circular, funnel-shaped pools, often with jagged
walls, filled with thermal water (fig. 19). Many of these pools appear to have formed in
hydrothermal explosions (Marler and White, 1975). Porkchop Geyser in Norris Geyser Basin,
Excelsior Geyser in Midway Geyser Basin, and Seismic Geyser in Upper Geyser Basin have
produced well-documented historical explosions (Marler and White, 1975; Fournier, 1989;
Whittlesey, 1990). Geologic research has identified many large craters formed before the time of
historical records, including a number at and near the north edge of Yellowstone Lake (fig. 20:
Mary Bay, Indian Pond, and Turbid Lake; also see fig. 24). If similar large explosions were to
occur today in that vicinity, they could threaten infrastructure and visitor safety along the East
Entrance Road, at Fishing Bridge and as far away as Lake Village. Geologic and historic evidence
in Yellowstone and elsewhere suggests that such hazardous activity could last days, weeks or even
years.
Browne and Lawless (2001) provide a lengthy summary of current knowledge of
hydrothermal “eruptions” around the world, preferring the word eruption over explosion to
reinforce the idea that the events were part of a continuum extending from geysering (no rocks
ejected) to very large rock-hurling eruptions creating craters hundreds to thousands of meters
across. While acknowledging the terminology of Browne and Lawless (2001) but to avoid
confusion with magmatic eruptions, we continue to use the term hydrothermal “explosions” in this
hazards assessment for events that can form craters or eject rocks.
We follow Browne and Lawless (2001) in distinguishing hydrothermal eruptions/explosions
from other types of ground water eruptions such as:

13
Phreatic eruption: An eruption of steam, water and rock caused when cool ground water is
heated rapidly to its boiling point by magmatic heat but no magma is actually erupted.
Phreatomagmatic eruption: Similar to a phreatic eruption but with evidence for eruption of
magma as well as fragments of the confining rock.
Magmatic-hydrothermal eruption: An eruption caused by magma heating a pre-existing
hydrothermal system; the magma may or may not reach the surface as a part of the ejected
materials. An excellent example occurred in New Zealand in 1886, when intrusion of basaltic
magma into a long fracture system caused hydrothermal explosions as far as 20 km away from the
lava vent (Hedenquist and Henley, 1985; Simmons and others, 1993).
In contrast, hydrothermal eruptions/explosions occur in pre-existing hydrothermal systems
without the proximate influence of magma. It appears that all explosions at Yellowstone since the
latest glaciation, roughly the past 16,000 years, are true hydrothermal explosions with no direct
participation of magma.
Pressures, temperatures, and fluids in geothermal systems
Understanding the causes of hydrothermal explosions requires understanding the structure
and dynamics of geothermal systems. Such systems are regions of anomalous heat flow in the
shallow crust of the Earth. Geothermal systems can be divided into two distinct types: hot-water
(or liquid-dominated) systems and vapor-dominated systems (White and others, 1971).
In a liquid-dominated system, the Earth’s subsurface is saturated with hot water, which is
located in interconnected pore spaces and fractures. Liquid-dominated systems can have two types
of surface manifestations: a) hot springs, which can represent the direct outflow of hot water from
the geothermal reservoir, and b) fumaroles, which represent steam and gas boiled off the reservoir.
At Yellowstone National Park, the hot springs of liquid-dominated systems are typically neutral or
slightly alkaline, Cl-rich, and are saturated with silica (SiO2). As these waters cool, they precipitate
amorphous silica as deposits called siliceous sinter. Waters that precipitate silica are common in,
but are not limited to, the Upper and Lower Geyser Basins, the Norris Geyser Basin, the West
Thumb Geyser Basin (Fournier, 1989) and sublacustrine hydrothermal vents in Yellowstone Lake
(Shanks and others, 2005; Shanks and others, in review).
Vapor-dominated systems lack sufficient water for complete liquid saturation of the
reservoir, resulting in low-density steam and gas being the interconnected phase. Vapor-dominated
reservoirs can still contain an appreciable amount of liquid, which remains within pore spaces in
the host rock. Surface manifestations above vapor-dominated reservoirs typically consist of “acid-
sulfate” features such as fumaroles, acid pools, and mud pots. Hot springs issuing neutral waters
are usually absent. Instead, acid pools and mud pots form due to condensation and oxidation of
acid gases and steam in perched bodies of surface and near-surface water. Most of the large
thermal areas in the eastern part of Yellowstone National Park (e.g., Mud Volcano, Hot Springs
Basin, Sulphur Hills, Josephs Coat) formed above vapor-dominated geothermal areas and have
acid-sulfate soils and widespread alteration of surface rocks, commonly to clays (White and others,
1971; White and others, 1975).
The simplest geothermal reservoir at Yellowstone consists of ground water at its boiling
temperature for a given hydrostatic head (fig. 21). In these situations, pressure is dictated by the
weight of the interconnected column of subsurface and surface water. Under normal conditions,
the confining pressure is sufficient to prevent the water from boiling catastrophically. Geyser
eruptions occur when subtle boiling at the top of the column induces local depressurization and
consequent boiling further down the column. Conditions such as a landslide, earthquake, or dam-
break may cause a large and sudden decrease in pressure; geothermal water then may become
highly superheated relative to its boiling temperature at the new, lower pressure, causing rapid
conversion of some of the water to steam, forcing expansion, and triggering an explosion.

14
Hydrothermal explosions can be initiated more readily when pressures exceed hydrostatic. Ideally,
because the density of hot water is less than that of cold water, hydrothermal systems should be
slightly under-pressured with respect to the cooler surrounding ground water systems. However,
mineral-precipitation reactions, especially those forming silica, serve to clog permeable pathways
in the aquifer and can cause the geothermal waters to become isolated with respect to their
surroundings. Moreover, the presence of gas and steam can increase the pressure of the system. As
a result, many geothermal drill holes, including research holes drilled at Yellowstone in the 1960s
(White and others, 1975), are positively pressured with respect to the predicted hydrostatic gradient
(fig. 22).
An additional factor increasing the pressure within geothermal areas is the buildup of gases
such as carbon dioxide (CO2) and hydrogen sulfide (H2S), which do not condense upon ascent and
cooling and can build up in concentration beneath an impermeable caprock (Hedenquist and
Henley, 1985). At the vapor-dominated Ngawha system in New Zealand, for example,
extrapolations of deep gas pressures to near surface conditions could allow a hydrothermal eruption
that would lift and disperse about 130 m of overlying rock (Browne and Lawless, 2001). Thus,
vapor-dominated reservoirs also can experience hydrothermal explosions, either owing to buildup
of non-condensable gases or simply by release of superheated steam. In general, explosions from
vapor-dominated reservoirs are thought to be somewhat less dangerous than liquid-dominated
reservoirs because a given volume of steam will contain less potential energy than the same volume
of liquid at the same temperature. Even in vapor-dominated reservoirs, most of the potential
energy of the system resides within the residual liquid water (Browne and Lawless, 2001).
Mechanisms of hydrothermal explosion
The mechanisms for hydrothermal explosions recognized by Browne and Lawless (2001)
are summarized here in the context of the geothermal system at Yellowstone.
Pressures exceeding lithostatic: A hydrothermal explosion can occur if fluid pressure is
regulated by an impermeable caprock, such that pressures increase until they exceed the weight and
strength of the overlying rock. This mechanism is thought to be comparatively rare at geothermal
fields, where measured pressures are usually well below lithostatic, but eruptions from these fields
are relatively common. Moreover, the common hot springs and fumaroles at the surface above
geothermal fields imply that their caprock is not generally impermeable. At Yellowstone, it is clear
that areas affected by hydrothermal explosions are those having already-established reservoirs
connected to distinct surface expressions. Browne and Lawless (2001) infer that this mechanism is
most important above young geothermal reservoirs as they start to interact with overlying, as yet
unaltered, surface rocks. Phreatic eruptions at reactivating volcanoes also might occur when
transients in ground water pressures exceed the lithostatic load. Potentially, self-sealing due to
precipitation of minerals from hydrothermal fluids could decrease the permeability of hydrothermal
aquifers, causing pressures to approach lithostatic.
Slow accumulation of steam and/or gas: Pressures can more regularly exceed lithostatic if
steam is present in the system and can ascend, transmitting pressure to shallower regions. This is
thought to be the common mechanism for explosions in exploited geothermal fields (Browne and
Lawless, 2001) when withdrawal of geothermal fluid lowers subsurface pressures, thereby
triggering additional boiling. Because cooling of noncondensable gas has a negligible effect on its
volume, its presence will increase the likelihood of overpressure. Long-term drought and a drop in
ground water levels could cause a similar phenomenon wherein shallow steam pressures increase
with a decrease in the elevation of the ground water table (fig. 23, adapted from fig. 6 of Browne
and Lawless, 2001).
Rapid subsurface pressure release: Any geothermal system that follows the boiling point
with depth can be triggered into an eruption by a sudden release in pressure that causes flashing of

15
liquid water to steam. Such a pressure release could result from an earthquake, a landslide,
draining of a lake, deglaciation, or a hydrothermal fracturing event within the reservoir. At
Yellowstone, such events are known to have caused hydrothermal perturbations. The 1959 Hebgen
Lake earthquake (M=7.5) induced eruptions of some 289 springs, including 150 with no previous
record of geysering (Marler, 1973). Most of these eruptions were solely of liquid water and steam,
though Marler and White (1975) document the growth of a new feature, Seismic Geyser, whose
genesis involved a series of rock-hurling episodes. Muffler and others (1971) postulated that the
very late Pleistocene Pocket Basin explosion in the Lower Geyser Basin was initiated by drainage
of a postglacial lake, causing a pressure drop in the underlying geothermal reservoir.
Addition of magmatic heat or gas: Addition of external magmatic heat or gas yields what is
termed a magma-hydrothermal eruption. One might consider all heat at Yellowstone to be
ultimately magma-derived, but the term is used here to denote a rapid transfer of heat from a
shallow magmatic intrusion directly into the geothermal system. Though such a mechanism is
unlikely for the small shallow explosions most common at Yellowstone, its importance cannot be
ruled out for some of the earlier large hydrothermal explosions of the late Pleistocene and early
Holocene. If magma, however, did induce some of the large explosions, it did so without reaching
the surface.
Progressive flashing: Browne and Lawless (2001) conclude that this is the most common
type of hydrothermal explosion. It initiates close to the ground surface and works its way
downward with time as rocks fracture, causing the boiling front to move down, resulting in
increased boiling, brecciation, and depressurization of the underlying system. The model requires
that boiling water exists near the surface and overlies water at a boiling-point-for-depth temperature
gradient. As such, the initial confining pressure can be minimal, and open pools can be the site of
initiation. The eruption itself might be triggered by events like those described in the subsurface
pressure release model, but with the boiling water column destabilizing much closer to the surface.
Hydrothermal explosions in Yellowstone
At Yellowstone, a geologic record of hydrothermal explosions exists only for events
occurring after the most recent glaciation (ending about 16,000 years ago, Pierce and others, 2002),
which effectively erased evidence of any earlier such events. Evidence for eighteen large (>100-m)
hydrothermal explosions (table 3; fig. 24) is found within Yellowstone National Park (Morgan and
others, in review). Many large explosion craters are present on land, but others have been
identified in Yellowstone Lake (Wold and others, 1977; Morgan and others, 2003b; Morgan and
others, in press-b; Morgan and others, in review). Most of the largest explosion craters, such as
Mary Bay, Turbid Lake, Duck Lake, Indian Pond, and Pocket Basin (fig. 24), are found within and
along the margin of the 640-ka Yellowstone caldera. A few are present along the tectonically
controlled, north-trending Norris-Mammoth corridor. The hydrothermal-explosion craters appear
to be relatively shallow features affecting only the upper several hundred meters of substrata, which
typically have been previously affected by hydrothermal alteration (Muffler and others, 1971;
Morgan and others, in review). In Yellowstone, no large hydrothermal explosion is associated with
a volcanic event, and no evidence exists for a hydrothermal explosion triggering any volcanism.
Figure 25 displays historical hydrothermal-explosion sites identified through a literature
search. As with the prehistoric craters and deposits, the historical eruption sites are concentrated
within the caldera and the Norris-Mammoth corridor. Notably, the Upper and Lower Geyser
Basins hosted many of the observed events. Appendices 1 and 2 provide descriptions of some of
the historically observed explosions and the prehistoric explosion deposits studied.
The end result of most hydrothermal explosions is a crater, commonly water-filled,
surrounded by a berm of fragmental rocks with steep inward slopes and gentler outer slopes. The
deposits generally comprise pieces of hydrothermally altered or mineralized materials such as

16
siliceous sinter, mud, breccia, and subsurface lithologies such as lake, beach, or glacial sedimentary
rocks and rhyolitic rocks from deeper levels.
The individual fragmental deposits are quite varied and reflect the stratigraphy of
underlying rocks and the depths of rocks evacuated by the explosion, and they commonly record
complex, multiple-event histories. Silica, in the forms of quartz, chalcedony, opal, and amorphous
silica, is the most common cement of the explosion breccias and cross-cutting veins, but pyrite,
calcite, and zeolites also are common hydrothermal minerals.
The complexity of the deposits implies repeated explosion events over a wide range of
scales, with lithic fragments being ejected, falling back into or adjacent to the vent, and being
cemented with silica to form breccias. Multigenerational heterolithic breccias also are common,
indicating an extended process of repeated brecciation and hydrothermal cementation that likely
occurs in hydrothermal systems well below the explosion vents (Keith and Muffler, 1978; Morgan
and others, in review).
Factors contributing to hydrothermal explosions
Several factors contribute to the likelihood of hydrothermal explosions. Prior to the
eruption of Porkchop Geyser in 1989, Fournier and others (1991) had interpreted chemical
indicators as showing that the temperature of the water feeding the hot-spring pool had been
increasing with time. Increased temperature of the deep water would have had two primary effects.
First, it would have increased the amount of boiling and therefore steam production as the water
rose towards the surface. Second, it would have increased the amount of silica in the ascending
water and thus the supersaturation of silica as the water cooled during its rise. The first factor
would have increased pressure in the system as the steam/liquid ratio in the subsurface increased in
the relatively constant-volume system. The second factor would have decreased permeability in the
geyser’s conduit, potentially causing a decrease in the rate at which water could flow through the
system. Thus, increased heat to the system increased the likelihood of pressurization and
hydrothermal explosions.
Another influence on the likelihood of hydrothermal explosions is the depth of the vapor-
liquid interface above a liquid-dominated geothermal reservoir (fig. 23), as this depth controls the
pressure of any vapor reservoir near the surface. If rocks of low permeability overlie a vapor-
dominated region, then the pressure can rise if any or several of the following things should
change: a) increased heat supplied to the system, b) increased amount of gas or steam rising
through the system, or c) reduced ground water recharge to the system causing the geothermal
water table to fall. Any of these factors would cause an increase in the thickness of the steam
reservoir, resulting in greater pressures transferred toward the surface through the vapor/steam
reservoir. This process occurs commonly at geothermal wells, where gas must be “bled off” to
prevent displacement of water and lowering of the water level within the well. If gas or steam is
allowed to accumulate in idle geothermal wells, the results can be disastrous. Because the ground
surface at Yellowstone is generally permeable above vapor-dominated areas, allowing abundant gas
flux (Werner and Brantley, 2003), this mechanism may not be a leading cause of explosions at
Yellowstone but might be so on occasion.
Rapid pressure reduction is commonly invoked as a cause of hydrothermal destabilization.
Withdrawal of fluids from geothermal wells is documented to have lowered subsurface pressures
and induced boiling and hydrothermal explosions in geothermal fields (Scott and Cody, 2000).
Earthquakes, deglaciation, or lake drainage all could cause sufficient depressurization of a boiling
aquifer to cause hydrothermal explosions at Yellowstone (Muffler and others, 1971; Morgan and
others, in review). Bargar and Fournier (1988) demonstrated that parts of the geothermal reservoir
were superheated by 20-50°C following deglaciation, at about 12 to 15 ka. This is presumably
because while glacial ice was present, areas beneath the ice-rock contact were pressurized

17
compared with ice-free conditions, allowing a higher boiling temperature for H2O. After
deglaciation, considerable boiling would be necessary to re-equilibrate the water column to the
new, lower-pressure conditions. Although this temporary instability would not likely be a direct
trigger for hydrothermal explosions, it could be a possible contributing factor; whether any
explosions resulted directly from deglaciation and attendant depressurization is not known.
Earthquakes, extremely common at Yellowstone, are also known to have strong effects on
geothermal features (Watson, 1961; Marler, 1964; Marler and White, 1975; Husen and Smith,
2004). Some of the large prehistoric hydrothermal explosions could have been triggered by
destabilization due to passage of large-amplitude seismic waves that dynamically increase local
stress in hydrothermal reservoirs.
Although several factors can be identified as potential triggers for hydrothermal explosions
at Yellowstone, the reality is that there are only sparse observational or scientific monitoring data
on past explosive events. Geologic evidence for triggering mechanisms for past events is rare and
ambiguous though current studies are aimed at detecting such information where it may exist.
Toxic gases
After steam, carbon dioxide (CO2) is the most common constituent of volcanic gas and can
be emitted in sufficient quantities to pose a hazard at many volcanic and geothermal systems
around the world (Baxter, 2005). After subtracting steam, CO2 typically constitutes 95 to 98% of
the gas emitted from Yellowstone’s fumaroles and bubbling pools (Werner and Brantley, 2003).
Carbon dioxide is non-toxic in low concentrations and makes up about 0.038% of the Earth’s
atmosphere. However, because it is about 50% heavier than normal air, it can accumulate to much
higher concentrations in soils and low or protected areas such as valleys and caves. Carbon dioxide
concentrations of >10% are toxic to humans and animals. When air contains over 20-30% CO2,
even a few breaths can quickly lead to unconsciousness and death from acute hypoxia, severe
acidosis, and respiratory paralysis (Hill, 2000). Hypoxia is a condition in the body resulting from
the displacement of oxygen such that it inhibits normal metabolism. Acidosis occurs when CO2
acidifies the blood, causing irreversible cellular damage. In the Yellowstone region, the local
concentration of CO2 to potentially toxic levels is generally only temporary and is restricted to
confined or topographically low areas of hydrothermal activity, generally under windless
conditions.
Volcanoes commonly emit acid gases like sulfur dioxide (SO2) and hydrogen chloride
(HCl). For example, at the volcanoes Kilauea in Hawaii and Masaya in Nicaragua, these acid gases
form aerosols that are a chronic hazard to both local vegetation and human populations (Baxter,
2005). At Yellowstone, the hydrothermal system and its host rocks act as a buffer that absorbs and
neutralizes acid gases, forming hydrogen sulfide (H2S) gas and soluble sodium chloride.
Additional H2S may rise directly off the magma. The concentration of H2S is typically about 20 to
200 times less than that of CO2, but its toxicity is much greater. Though H2S has an extremely
strong “rotten-egg” odor at levels of only a few parts per billion (ppb), concentrations of more than
10 ppm in the air will rapidly deaden the human sense of smell to its presence (Mandavi, 2005).
Concentrations of 100 ppm can cause severe eye and throat irritation, and at concentrations
exceeding 700 ppm loss of consciousness and death can occur rapidly. Hydrogen sulfide forms a
complex bond to iron in mitochondrial cytochromes, thereby arresting aerobic metabolism in an
effect similar to cyanide toxicity (Milby and Baselt, 1999).
The Hazards
The geologic setting, geophysical activity, and hydrothermal systems reviewed above
provide the framework within which to consider potential hazards arising from any future volcanic,

18
hydrothermal, or gas-emission activity. For each of these categories of potential hazards,
discussion is organized in terms of different types of activity that might develop as the Yellowstone
magmatic-tectonic-hydrothermal system continues to evolve.
Volcanic-eruption hazards
As is characteristic of many large continental magmatic systems, eruptive activity in the
Yellowstone Plateau volcanic field is highly episodic and involves long periods of time between
eruptions; many of the eruptions are quite large. Such systems are particularly difficult to evaluate
in terms of the probabilities of hazardous events and the consequent risks to people and resources.
In order to bring some coherence to this discussion, it is organized by considering first the smaller,
more likely future volcanic eruptions and proceeding to the larger and potentially most destructive
but least likely events.
Basaltic eruptions
As noted earlier, basaltic lavas have erupted around the margins of the active, mainly
rhyolitic Yellowstone Plateau volcanic field throughout its evolution. The absence of basalts from
within the rhyolitic source areas is interpreted to reflect the trapping within the crust of any basaltic
magmas that might have intruded from zones of partial melting in the upper mantle beneath crustal
rhyolitic magmas of lower density (Christiansen, 2001). Only after about a million years have
basalts erupted through the cooled, crystallized, and fractured upper-crustal magmatic sources of
the first and second rhyolitic cycles; no basaltic vents, however, occur within the third-cycle
Yellowstone caldera. A few small outcrops of basalt do occur on the northwest caldera wall near
Purple Mountain (Christiansen and Blank, 1974), but they are erosional remnants of lavas that
flowed down the steep slope from vents farther north. Additionally, some rare quenched inclusions
of basaltic magma were found within the basal part of the rhyolitic West Yellowstone flow near the
crest of the Madison Plateau west of Little Firehole Meadows (R. L. Christiansen and H. R. Blank,
Jr., unpubl. data), suggesting that basaltic magmas from lower-crustal levels might have played a
role in mobilizing some intracaldera rhyolitic magmas for eruption.
Most postcaldera basalts of the Yellowstone area are glaciated erosional remnants of once-
larger flows (fig. 10). Extrapolation to the likely outlines of initial distributions suggests that most
individual basaltic eruptions covered areas of less than about 5 km2
and did not exceed 0.1 km3
in
volume. The largest single basaltic flow field, however, the Falls River Basalt, extends from the
southwestern caldera margin to Henrys Fork of the Snake River, a distance of about 60 km. The
Falls River Basalt flow field may have covered 900 km2
and may account for an eruptive volume of
nearly 20 km3
. At least two vents and possibly more fed the flow field, only one of which is now
exposed (fig. 10). The next largest basaltic flow field near Yellowstone National Park, the Gerrit
Basalt covering about 100 km2
in the area of Island Park, west of Yellowstone, erupted from at
least 13 vents. Basalts cover the Eastern Snake River Plain west of Island Park, and at least 4 vents
for basalts of the Snake River Group are within or immediately adjacent to the basin of Island Park
(Christiansen, 1982).
The postcaldera basalts of the Yellowstone region—generally pahoehoe flows—are mainly
olivine tholeiites having a range of K2O contents (but commonly <0.5%). Lavas producing such
flows would be expected to have low viscosities and to erupt rapidly. Probably most of the basalts
erupted in events lasting no more than a few weeks to a few months, but a large flow field like the
Falls River Basalt might have accumulated in multiple eruptions, each lasting many months. Most
of the recognized basaltic vents are localized agglutinated scoria accumulations or small lava
shields, but some basalts may have originated as dike-fed fissures. A few formed cinder cones.

19
A total of 33 postcaldera basaltic vents (fig. 10) have been recognized or inferred in the area
immediately surrounding the Yellowstone caldera (Christiansen, 2001; Smith and Bennett, 2006).
Of these, 17 are within or adjacent to the Island Park basin; the others are scattered around all
sectors of the Yellowstone-caldera margin. Because of their generally scattered distribution, most
of the vents probably remain preserved at the surface, but some might have been buried by younger
materials or not recognized during geologic mapping. If it is assumed that 80 percent of the actual
vents have been recognized, there could have been as many as 40 basaltic eruptions around the
periphery of the Yellowstone caldera in the past 640,000 years. Of these about half occurred in
Island Park. No postglacial basaltic eruptions have been recognized, indicating that none has
occurred within the past 16,000 years.
On the basis of the foregoing, the average period between basaltic eruptions in the area
around Yellowstone National Park during post-Lava Creek time is about 16,000 years. The
average annual probability (i.e., the number of events divided by the time period, in years, during
which they have occurred) of a basaltic eruption occurring somewhere around the periphery of the
Yellowstone Plateau volcanic field is therefore 6x10-5
. However, it is unclear whether the basaltic
eruptions, like some of the Yellowstone rhyolitic eruptions, may have been clustered in time; if so,
the long-term average recurrence period may have little direct bearing on future eruption
probabilities. The most likely location for any such future basaltic eruption is within the basin of
Island Park but could be anywhere else within a band about 40 km wide surrounding the
Yellowstone caldera. Any such eruption is most likely to last between a few weeks and several
months. It is possible but unlikely that basalt could erupt from within the caldera; if such an event
were to occur, however, it would signal the demise of the large Yellowstone-caldera rhyolitic
magma chamber.
The principal hazard likely to result from a basaltic eruption around the periphery of the
Yellowstone caldera would be coverage of an area of several square kilometers by lava, one to a
few tens of meters thick. In addition, basaltic ash and cinders from the eruptive vent might blanket
areas of many hundreds of square kilometers to depths of a few meters to a few centimeters,
decreasing in thickness outward from the vent in directions determined by the prevailing winds. If
a basaltic vent were to emerge from beneath shallow water or a large area of saturated ground,
phreatomagmatic eruptions could produce pyroclastic surges within the proximal area that could
blast down trees and cause other similar destruction.
In addition to any primary hazards of lava inundation and ash blanketing, there are likely to
be secondary hazards from any basaltic eruption in the Yellowstone region. In particular, such an
event would be likely to start fires around an advancing flow front, particularly under dry
conditions. Debris flows or floods could be triggered by the melting of any significant snow pack
or by temporary blockages of major drainages and subsequent release of floodwaters as the
blockage was undermined or overtopped by rising waters.
Given the ongoing YVO monitoring program, it is likely that there would be recognizable
premonitory indicators of any impending basaltic eruption. In particular, multiple shallow
earthquake swarms focused in a small, probably linear area would be likely to be followed or
accompanied by volcanic tremor, as has been observed at many basaltic volcanoes as they prepare
to erupt. It is likely that surface ground cracks would open in the immediate stages prior to any
basaltic eruption as a dike approached the surface. Because YVO deformation monitoring is
focused on the Yellowstone caldera, initial uplift associated with the shallow intrusion of basaltic
magma peripheral to the caldera might not be recognized quickly.
The emission of magmatic gases to the surface would be a major indicator of impending
eruptive activity but might be quite difficult to recognize in the presence of Yellowstone’s massive
hydrothermal system, which tends to scrub out magmatic gases passing through it (Symonds and
others, 2001). Nevertheless, any locally increased emissions of CO2 or H2S should be monitored

20
closely; any appearance of SO2, presently absent at Yellowstone, would be especially indicative
that the hydrothermal system was becoming dried out by shallow magmatic intrusion. Even
without SO2, significant localized increases in the ratio of sulfur gases to carbon gases would
suggest the possibility of magmatic gas reaching shallow subsurface levels.
Because no basaltic eruptions have occurred in more than 16,000 years at Yellowstone,
there are no well-established magmatic pathways. Thus, it is most likely that premonitory
seismicity would be sufficiently prominent and of long enough duration to allow temporary
monitoring of ground deformation and gas emission to provide additional information for
interpreting possible locations and the nature of any such eruptive event. Among the earliest
indicators might be relatively deep long-period seismicity. The most immediate precursors
probably would occur as a shallow intrusion entered the hydrothermal system, generating very
active short-period seismicity and possibly hydrothermal explosions. However, the distinction
between an impending basaltic or small rhyolitic eruption might be difficult to evaluate before the
initial venting.
Because the basaltic lava flows of Yellowstone are virtually all tube-fed pahoehoe, their
advance would probably be slow enough to allow mitigating measures to be taken except, perhaps,
in areas close to erupting vents.
Large rhyolitic eruptions
At least 17 large rhyolitic lava flows, most of them with volumes of 10 km3
or greater, have
erupted within the Yellowstone caldera during about the past 170,000 years (Christiansen, 2001).
Stratigraphically they belong to the Central Plateau Member of the Plateau Rhyolite. Each of these
lava flows extruded through one of two linear vent zones that cross the caldera along the
extrapolated positions of extracaldera tectonic fault zones (figs. 8, 11), and activity has been
essentially contemporaneous on both zones. The largest of these lava flows cover areas greater
than 350 km2
and have volumes greater than 30 km3
(table 2). The constructional topography
formed by these flows is represented by the Pitchstone, Madison, and Central Plateaus (figs. 1 and
26). The Pitchstone-Madison Plateau alignment lies on the extrapolated position of the Teton
normal-fault zone (fig. 11) and extends to the southwest edge of the tectonic West Yellowstone
basin (fig. 8). The Central Plateau alignment (fig. 8) lies on an extrapolation of both the Sheridan
normal-fault zone (fig. 11) and the extracaldera Norris-Mammoth corridor (fig. 10).
As these young intracaldera rhyolite flows are petrographically, chemically, and isotopically
distinct from early intracaldera rhyolites (Hildreth and others, 1984; Hildreth and others, 1991) and
have an age range separated by about 80,000 years from the youngest known earlier intracaldera
flows, they are considered here together as a group separate from the older postcaldera lavas. The
hazard potential for a possible future intracaldera eruption may be best reflected in the
characteristics of this group of voluminous rhyolites of ~170 ka and younger.
Available K-Ar and 40
Ar/39
Ar dating of rhyolitic flows of the Central Plateau Member (table
2; figs. 9, 26) indicates that they erupted in a few major episodes. The earliest of these episodes
probably began with the Mallard Lake flow that erupted from the Central Plateau vent alignment
and covered the eastern part of the Mallard Lake resurgent dome (Christiansen, 2001). It was either
accompanied by or immediately followed by renewed uplift of the Mallard Lake dome.
Subsequent eruptions, within a period of less than 10,000 years, produced the Dry Creek, West
Thumb, and Mary Lake flows along the Central Plateau vent zone and the Buffalo Lake flow along
the Madison Plateau vent zone (fig. 9). Additional rhyolitic lavas, now buried, may well have
erupted during this time from either or both of those zones. In addition to these lavas, at least one
major pyroclastic eruption occurred along the Central Plateau vent alignment to produce the tuff of
Bluff Point. Although its volume cannot be reconstructed convincingly because of erosion and
burial by younger lavas, eruption of this pumiceous pyroclastic unit was sufficiently large to

21
produce a source caldera 10 km in diameter, now represented by West Thumb, the westernmost
basin of Yellowstone Lake (fig. 8). The eruptive volume almost certainly was several tens of km3
.
The actual time span represented by eruptions of these oldest units of the Central Plateau Member
is not defined precisely. The nominal span of weighted-mean ages for the dated lava flows and tuff
of this group is 173±11 to 160±3 ka, but disagreements between individual age determinations and
the stratigraphic order interpreted from geologic mapping suggest that some of the isotopic ages are
incorrect. The group mean age and standard deviation of the individual weighted-mean isotopic
age determinations on all of these units is 167±5 thousand years, and all the weighted-mean ages of
the individual units of this group overlap that range. Thus, the range might be a closer
representation of the actual span of time involved in erupting them than the nominal span of
individual age determinations.
Distinct in time but only shortly after the first group of eruptions along the Central Plateau
vent alignment (fig. 8) were the Aster Creek, Elephant Back, Spruce Creek, and Nez Perce Creek
lava flows (fig. 9). The undated Spring Creek flow may have vented on the Madison Plateau vent
alignment during this same episode. At least one significant pyroclastic unit, the tuff of Cold
Mountain Creek, is also interstratified among lavas erupted along the Madison Plateau alignment
although its source location is not known. This tuff may represent early pyroclastic activity from
the vent for a large lava flow of the Madison Plateau. The age of this group of rhyolitic eruptions is
about 150±5 ka, calculated similarly to that of the earlier group.
Another relatively brief episode of large-volume rhyolitic eruptions within the Madison
Plateau vent zone formed the Summit Lake, Bechler River, and West Yellowstone flows (fig. 9);
the smaller Douglas knob and Trischman Knob domes also probably erupted during this episode,
possibly as late-stage vent domes of the Bechler River flow (Christiansen, 2001). The area covered
by flows of this age group is smaller than for the older groups, and all flows erupted during this
sequence probably are represented by surface exposures. Nominal weighted-mean K-Ar ages of
these lavas range from 124±10 to 114±1 ka, but just as for the older groups, stratigraphic relations
suggest that the actual time span of eruption was short. A mean age and standard deviation of the
individual weighted-mean age determinations on these flows is 118±5 ka.
Yet another episode of relatively large rhyolitic lava eruptions vented along the Central
Plateau at about 102-103 ka, producing the Hayden Valley and Solfatara Plateau flows (fig. 9).
The age of these flows, calculated similarly to those of the older groups, is 103±1 ka.
The most recent episode of large intracaldera rhyolitic lava eruptions is represented by the
adjacent Grants Pass and Pitchstone Plateau flows (fig. 9), with weighted-mean ages of 72±3 and
79±11 ka, respectively. Both flows may well represent a single eruptive event, the linear vent for
the Grants Pass flow representing an early dike phase, and the Pitchstone Plateau flow issuing from
a longer-lived central-vent phase. The weighted-mean age and standard deviation of these flows is
76±5 ka.
Several factors suggest a model for systematic evolution for rhyolitic eruptions of the
Central Plateau Member that may have implications for related volcanic hazards. Volumes of the
Central Plateau lavas (table 2) were calculated from geologic maps using a cut-and-fill estimator in
ArcInfo, a Geographic Information System; those calculations proceed by estimating the original
extent and a computer-generated extrapolation of a flat bottom to each flow, resulting in a
minimum-volume estimate and, thus, probably an underestimate of the total volume of all the
flows. The aggregate volume of the oldest group, at 167±5 ka, is by far the greatest, at least 138
km3
. That volume, however, is possibly even greater, ~400 km3
as suggested by a separate
calculation of the total volume for all Central Plateau Member lavas of >600 km3
, with the
difference between these calculations listed as “unobserved units” in table 2. (An earlier estimate
of the total volume of the Central Plateau Member by Christiansen (2001) was 900 km3
). The
aggregate volume of the second group of lavas, at 150±5 ka is at least 62 km3
, and that of the next

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